This application claims the priorities of Japanese Patent Applications No. 4-70395 and No. 4-70396 both filed Feb. 20, 1992, which are incorporated herein by reference.
The present invention relates to a hard alloy, such as cermet or cemented carbide, which exhibits excellent wear resistance and fracture resistance when used as cutting tools.
A known cermet which includes: a hard dispersed phase composed of carbonitride of titanium (Ti) or composite carbonitride of titanium and at lease one element of tantalum (Ta), tungsten (W), molybdenum (Mo), niobium (NBc), vanadium (V), chromium (Cr), zirconium (Zr) or hafnium (Hf); and a binder metal phase composed of at lease one metal of cobalt (Co), nickel (Ni), iron (Fe) or aluminum (Al) has hitherto been used in cutting tools for finishing cuts on steel or the like, whereas a known cemented carbide which includes: a hard dispersed phase composed of tungsten carbide (Wc) and optionally at least one compound of carbide, nitride or carbonitride which contains at least one element of titanium, tantalum, molybdenum, niobium, vanadium or chromium; and a binder metal phase composed of at least one metal of cobalt, nickel, iron or aluminum has hitherto been used in cutting tools for roughing cuts on steel, cast iron or the like.
Inasmuch as the aforesaid conventional hard alloy is a composite material comprised of the hard dispersed phase and the binder metal phase, compressive stress is intrinsically exerted on the hard dispersed phase while tensile stress is exerted on the binder metal phase upon the completion of sintering.
More specifically, cobalt, nickel, iron and aluminum, which serve as metals for defining the binder metal phase of the aforesaid hard alloy, have coefficients of thermal expansion of 12.36.times.10.sup.-6 /.degree.C., 13.30.times.10.sup.-6 /.degree.C., 1150.times.10.sup.-6 /.degree.C. And 23.13.times.10.sup.-6 /.degree.C., respectively. In contrast, since titanium carbide (TiC) and titanium nitride (TiN) have coefficients of thermal expansion of 7.42.times.10.sup.-6 /.degree.C. and 9.35 .times.10.sup.-6 /.degree.C., respectively, the coefficient of thermal expansion of titanium carbonitride (TiCN) defining the hard dispersed phase of the cermet, should have a value between them. Furthermore, with respect to the constituents defining the hard dispersed phase of the cemented carbide, the coefficient of thermal expansion of tungsten carbide is 5.2.times.10.sup.-6 /.degree.C. as measured in the a-axis direction, and 7.3.times.10.sup.-6 /.degree.C. as measured in the c-axis direction. Also, the coefficients of thermal expansion of tantalum carbide (TaC) and niobium carbide (NbC) are 6.29.times.10.sup.-6 /.degree.C. and 6.65.times.10.sup.-6 /.degree.C., respectively. Thus, in both cermet and cemented carbide, the coefficient of thermal expansion for the binder metal phase is greater than that for the hard dispersed phase, and hence the shrinkage of the binder metal phase, upon cooling after the sintering operation, becomes greater than that of the hard dispersed phase. Therefore, the binder metal phase shrinks in such a way as to encapsulate the hard dispersed phase therein, so that the hard dispersed phase undergoes compressive stress while the binder metal phase undergoes tensile stress. Thus, the compressive stress is retained in the hard dispersed phase of the resulting alloy, whereas the tensile stress is retained in the binder metal phase thereof.
In the case where the conventional hard alloy of the aforesaid construction is directly used to manufacture cutting tools, the cutting edges of the resulting tools are not only susceptible to chipping against the great impact to be exerted on the surfaces, but are also insufficient in wear resistance, thereby resulting in a very short tool life. In order to circumvent these problems, various specially developed sintering techniques have hitherto been applied to enhance the fracture resistance, or a hard coating has been formed on the surface of the tool to improve the wear resistance. However, since these measures require an increased manufacturing cost, the resulting cutting tools have become expensive.